Nanoimprint lithography (NIL) is a relatively new technology that finds use in the manufacture of planar micro- and nanodevices at low cost and with high productivity. Such effects result from the fact that in distinction from other known methods of generation and image transfer having nanodimensions, nanoimprint technology makes it possible to transfer a nanoimage as a whole and in one step instead of a multistage sequential transfer of image parts.
A similar task of one-step pattern image transfer is achieved by using modern optical steppers of high resolution. However, such technology is much more complicated and expensive than nanoimprinting.
Nanoimprinting consists of forming a pattern by means of a patterning lithography method, i.e., interference lithography, imprinting, or electron beam lithography (EBL) directly on the surface of the die or mold (template) and then reproducing the pattern by physical contact, e.g., by printing the pattern on a substrate, in particular on the resist layer of a substrate. This process may be repeated by using the same mold many times.
Depending on the nature of the imprinting materials, two main versions of NIL may be used, i.e., thermal NIL, which is based on use of the thermomechanical properties of resist films, and UV NIL, which is carried out with the use of a liquid resist material that is cured by ultraviolet light. Curing provides fixation of the image reproduced on the surface of the resist.
Furthermore, along with traditional nanolithography, NIL can also be used for forming functional surfaces or printable integrated devices by directly imprinting a functional resist material (also known in this field as ink).
One important application of NIL processes is the manufacturing of optical devices. For this purpose it is necessary to impart to the optical layers of such devices optical properties that can be controlled by chemical properties of the resist in which the pattern is formed. The main optical properties that should be controlled in the optical layers of the aforementioned devices are the refractive index and the level of optical losses.
Attempts have been made to control these properties. For example, a few works have reported the manufacturing of printed nanostructured surfaces and devices with a low refractive index (n≈1.45 to 1.55) in optical layers. The most successful example was reported last year with the introduction of nanoimprinted structures in the display of the Amazon Kindle Paperwhite tablet for front illumination (light-guide technology). However, the applications of printable devices are still very limited because the currently available NIL inks have relatively low refractive indices (n<1.62). The inks used in the above studies were limited to n<1.65, and there were no reports on a roll-to-roll process.
Several approaches have been investigated over the last five years to develop imprintable materials with high refractive index (n>1.65).
The work of Carlos Pina-Hernandez, et al, “A route for fabricating printable photonic devices with sub-10 nm resolution” published in Nanotechnology, Vol. 24, No. 6, January 13, discloses a novel and robust route for high-throughput, high-performance nanophotonics-based direct imprint of high refractive index and low-visible-wavelength absorption materials. Sub-10 nm TiO2 nanostructures are fabricated by low-pressure UV imprinting of organic-inorganic resist materials. Postimprint thermal annealing allows optical property tuning over a wide range of values. For instance, a refractive index higher than 2.0 and an extinction coefficient close to zero can be achieved in the visible wavelength range. Furthermore, the imprint resist material permits fabrication of crack-free nanopatterned films over large areas and is compatible for fabricating printable photonic structures.
One strategy is based on the use of inorganically cross-linked sol-gel materials, which due to their high thermal, chemical, and mechanical stability, are well suited for forming photonic functional layers. Furthermore, the optical properties of these materials are easily tunable (G. Rizzo, et al, “Sol-gel glass from organic modified silicates for optics applications,” J. Sol Gel Sci. Techn. 26, 1017 (2003); M. Li, et al, “Large area direct nanoimprinting of SiO2—TiO2 gel gratings for optical applications,” J. Vac. Sci. Technol. B 21(2), 660 (2003); and M. Okinaka, et al, “Direct nanoimprint of inorganic-organic hybrid glass,” J. Vac. Sci. Technol. B 24(3), 1402 (2006).
Previous studies have demonstrated the patterning of sol-gel materials by thermal nanoimprinting, but they were limited by time-consuming and complex procedures (high pressure, temperatures, and plasma treatment) (see, e.g., C. Marzolin, et al., “Fabrication of glass microstructures by micro-molding of sol-gel precursors,” Adv. Mater. 10(8), 571 (1998); and S. H. Um, et al, “Direct imprinting of high resolution TiO2 nanostructures,” Nanotechnology 21, 285303 (2010)].
It was recently demonstrated that hybrid organic-inorganic materials based on sol-gel chemistry offer a powerful alternative solution that overcomes these limitations. The obtained polymerized metal-organic materials demonstrated the fabrication of printable photonic devices into titania films with a refractive index up to n=2 for the first time (see C. Marzolin, et al, “Fabrication of Glass microstructures by micro-molding of sol-gel precursors,” Adv. Mater. 10(8), 571 (1998); and S. H. Lim, et al, “Direct imprinting of high resolution TiO2 nanostructures,” Nanotechnology 21, 285303, 2010)
It was shown in the work of Carlos Pina-Hernandez, et al, (see above) and in International Patent Publication PCT/US13/72109 that the process is suitable for high-resolution patterning with the replication of sub-10 nm structures. More interestingly, the process is suitable for fabricating printable photonic devices. The chip contains an on-chip demultiplexer, multimode ridge waveguides, and a light splitter. Each component is successfully measured. These results are state-of-the-art for imprinting high-refractive-index materials. However, the NIL ink used in the above work required a long postannealing step and the use of a high temperature; however, in view of the presence of a sol-gel composition, high temperatures are not suitable for nanoimprinting.
The second strategy consists of developing a resist material based on a polymer matrix doped with nanoparticles (most of the time TiO2). Based on a similar approach, previous works have reported the fabrication of transparent films with different polymer matrices (no thiol-ene) and a refractive index up to 1.8 (see H. I. Elim, et al, “A Novel Preparation of High-Refractive-Index and Highly Transparent Polymer Nanohybrid Composites,” J. Phys. Chem. B, 2009, 113, 10143-8; P. Tao, et al, “Polymer 54” (2013) 1639-1646; and Arfat Pradana, Christian Kluge, and Martina Gerken, “Tailoring the refractive index of nanoimprint resist by blending with TiO2 nanoparticles,” Optical Materials Express, Vol. 4, Issue 2, pp. 329-337, 2014.)
Thus, the chemical composition of ink, its chemical properties, and its adaptability for treatment in the course of nanoprinting are important factors that affect the quality of the final product.
Also known in the art of imprinting is the use of a thiolene resist, which has the advantage of imprinting and cross-linking in the presence of O2. The thiol-ene reaction (also alkene hydrothiolation) is an organic reaction between a thiol and an alkene. The reaction product is an alkenyl sulfide.
In this connection, reference can be made to the works of E. C. Hagberget, et al, “Effects of Modulus and Surface Chemistry of Thiol-Ene Photopolymers in Nanoimprinting,” Nano Letters, Vol. 7, No. 2 (2007); and S. Khire, “Formation and Surface Modification of Nanopatterned Thiol-ene Substrates using Step and Flash Imprint,” Advanced Materials, 3308-3313 (2008).
Chemically speaking, the aforementioned authors describe the following composition: thiol-pentaerythritol tetrakis-(2-mercaptopropionate) (PTMP); the ene component selected from (T1) 1,4-cyclohexanedimethanol divinyl ether (E1), (T2) 2-ethyl-2-(hydroxymethyl)-1,3-propanediol triacrylate (E2); (T3) 2,4,6-triallyloxy-1,3,5-triazine (E3) 1375; (T4) [50%] E1+[50%] bisphenol (E4) 460 E4-ethoxylated(2) bisphenol A dimethacrylate; (T5) [50%] E2+[50%] bisphenol (E4) 425; and (T6) [50%] E3+[50%] bisphenol (E4) 1800; and a photoinitiator, as an optional component, which was unidentified in the publication. All of these works involve the use of thiol-ene with a low refractive index (n<1.60).
However, the authors did not mention optical properties since their goal was only to develop an imprint resist with cross-linking in the presence of O2.
It is known that thiol and vinyl monomers with a functionality of 2 or higher can be used to create cross-linked materials. The ability of thiol-ene chemistry to photocross-link in the presence of air makes this approach very attractive for the NIL process when compared to materials based on radical polymerization, which are inhibited by oxygen. However, there are only a few works in the literature that have reported the direct imprinting of thiol-ene polymers with a resolution down to 100 nm (see above).
Another advantage of the above-mentioned materials is that they do not require postannealing, and they allow reaching a refractive index up to 1.7 at 546-nm wavelength. The relatively low viscosity (<0.1 Pa·s) of a thiol-ene polymer makes it suitable for the roll-to-roll imprint process. For the reasons mentioned above and taking into account a wide variety of available thiols and vinyl monomers, these materials become very attractive for types of ink used in the roll-to-roll imprint process. Another advantage of thiol-enes is their strong chemical resistance to different types of solvents as well as low gas permeability and good adherence to glass and metal, which makes thiol-enes ideal candidates for use as a functional ink for nanoimprinting.